1. Field of the Invention
This invention relates to thermoplastic compositions of a polyphenylene ether resin that have been stabilized against ultraviolet light radiation by the inclusion of a combination of a hindered amine light stabilizing (HALS) compound, a hydroxy-substituted benzophenone ultraviolet light absorbing (uv screener) compound, and a cyclic aliphatic epoxy synergist.
2. Description of the Prior Art
The utilization of engineering plastics has been extended to a variety of products and component parts, taking the place of more conventional structural materials such as wood, metals and glass. Many such applications involve the prolonged exposure of the plastic to sunlight or bright indoor light under conditions of ordinary use. For instance, more plastics are now being used as component parts on the exterior of automobile bodies, or as housings for office business machines. Certain plastics, such as aromatic polycarbonates, exhibit good stability against the photo-deteriorative effects of such ultraviolet light radiations. Other plastics, such as polyphenylene ether resins, are inherently less stable under those conditions, and they tend to undergo chemical and physical changes which are manifested typically by changes in color. To be more specific, the surface of the plastic article tends to become more yellow or yellowish, and this is especially apparent if the object is originally colorless or pigmented with a lighter color. This yellowing, or discoloration, is undesirable, chiefly for esthetic reasons.
The art has developed various means to stabilize polyphenylene ether resin compositions, and articles made from them, against such adverse effects. In general, compounds which function as ultraviolet light (UV) stabilizers are coated on the surface of the plastic object, impregnated into the surface, or incorporated into the plastic composition itself.
U.S. Pat. No. 4,288,631 (Ching) describes derivatives of 3-alkylhydroxybenzophenones, such as 2,4-dihydroxy-3-methylbenzophenone, as superior UV stabilizers for a variety of organic thermoplastic substrates, including poly(2,6-dimethyl-1,4-phenylene oxide). Only small amounts, from about 0.1 to 1 percent by weight, need be used. The photostability of 2,4-dihydroxy-3-methylbenzophenone versus 2,4-dihydroxybenzophenone is studied in one of the Examples and found to be visibly superior by showing no yellowing after over a 20-minute photolysis.
U.S. Pat. No. 3,640,928 (Murayama, et al.) discloses that many synthetic polymers can be stabilized against photo-deterioration by incorporating a small amount, 0.01 to 5.0 percent by weight, of certain piperidine derivatives. Typically, these are polyalkylpiperidine compounds, containing four methyl groups substituted on one or more piperidinyl rings.
U.S. Pat. No. 4,198,334 (Rasberger) indicates that certain substituted esters and amides derived from 4-hydroxy- and 4-amino-polyalkylpiperidines and hydroxymalonic acids are excellent stabilizers for organic polymers, especially polyolefins, but also polyphenylene oxides, protecting them against light as well as thermal oxidative degradation.
The prior art has also taught that certain combinations of compounds drawn from different chemical classes are possible for effecting UV light stabilization in polymers.
U.S. Pat. No. 4,315,848 (Dexter, et al.) reveals that the combination of a benzotriazole UV absorber with a hindered amine light stabilizer, e.g., a polyalkylpiperidine, is particularly efficacious in protecting thermoset and thermoplastic acrylic resin automotive finishes and enamels, but can also be used with a variety of organic polymers, including polyphenylene oxides.
Still other combinations are described by Patel and Usilton, in the article entitled "Ultraviolet Stabilization of Polymers: Development with Hindered-Amine Light Stabilizers", Stabilization and Degradation of Polymers, Advances in Chemistry Series, No. 169, American Chemical Society, Wash., D.C., 116-132 (1978). The authors studied the UV light stabilizing effects on polymeric substrates with bis(2,2,6,6-tetramethyl-piperidinyl-4) sebacate, as an example of a hindered amine light stabilizer, and compared it with other light stabilizers, including various benzotriazole compounds, a 4-hydroxybenzoate compound, and 2-hydroxy-4-n-octoxybenzophenone. Also tested was the effect of various pairings, including combinations of the aforementioned hindered amine with the benzotriazoles, the benzoate and the benzophenone, respectively. The experiments were directed to polyolefins, such as high density polyethylene and polypropylene, styrenic polymers, and aromatic polyesterpolyurethane. The article notes that significantly improved UV light stability occurs with bis(2,2,6,6-tetramethyl-piperidinyl-4) sebacate, and that synergistic performance of this compound with o-hydroxyphenyl benzotriazoles is apparent.
More recent developments have involved the use of combinations of certain alkoxy-substituted or alkyl ether-substituted 2-hydroxy-benzophenones with polyalkyldipiperidines as UV stabilizers in polyphenylene ether resin blends, with synergistic results being manifested. These are described in copending applications Serial No(s). 564,420 (Peter H. C. Shu), filed Dec. 22, 1983, now U.S. Pat. No. 4,555,538 and 570,330 (Richard C. Bopp), filed Jan. 13, 1984, now abandoned respectively, which have been assigned to the same assignee as herein.
Separately, the use of a cyclic aliphatic epoxide (Ciba-Geigy's ARALDITE CY179) in conjunction with 2,2,6,6-tetramethyl dipiperidinyl sebacate (Ciba-Geigy's TINUVIN 770, a hindered amine light stabilizer) has previously been investigated as a UV light stabilizer in blends of polyphenylene ether resin and a high impact polystyrene resin. The combination was found to provide substantial improvement in UV performance as determined by several UV tests, including window exposures, xenon arc under glass, and low intensity fluorescent exposures. In one test, this combination, in a 1:1 weight ratio, proved to be more effective by almost 50 percent than a 2:0.5 weight ratio of 2-hydroxy-4-n-octoxybenzophenone (American Cyanamid's CYASORB UV 531) and 2,2,6,6-tetramethyl dipiperidinyl sebacate (TINUVIN 770).